In a photonic network, a light amplifier is an important device to elongate a transmission distance of a light signal. In the photonic network, the WDM (Wavelength Division Multiplexing) transmission using a plurality of wavelengths is general for the transmission of a large amount of data in the network. In case of the WDM transmission, the light amplifier amplifies the light signal of the plurality of wavelengths. Here, it is thought of that the light power of the light signal which is supplied to the light amplifier changes due to increase or decrease of the number of wavelength paths on the network. For example, when the number of wavelength paths for the light signal which is supplied to the light amplifier is decreased, the light power of the light signal which is supplied to the light amplifier decreases. In this case, it is necessary to decrease the light power of the light signal which is outputted from the light amplifier, according to the supplied light signal.